This invention is generally directed to processes for the preparation of polymer particles, and to processes for the preparation of toner compositions, including dry and liquid toners. More specifically, the present invention is directed to the preparation of particles, including toner particles by, for example, dissolving a polymer in a solvent containing a block or graft steric stabilizer, and adding to the resulting mixture a liquid, or nonsolvent within which the polymer is insoluble or substantially insoluble. In one embodiment of the present invention, a polymer is dissolved in a solvent together with a stabilizer, such as a block copolymer, a triblock copolymer, a graft copolymer, or mixtures thereof followed by the addition of a nonsolvent for the polymer permitting the precipitation of the polymer product into particles suitable in some instances for imaging processes. One embodiment of the present invention comprises the precipitation of a polymer, including a homopolymer, or a copolymer A from a solution containing a suitable sterically stabilizing block or graft copolymer with A (or A-compatible) segments bonded chemically to B segments by the addition of a nonsolvent for A which is also a solvent for B, thereby providing a sterically stabilized dispersion of polymer particles, which polymers can be isolated by, for example, filtration or centrifugation. The aforementioned product particles can be formulated into toners by adding thereto pigments, dyes, colorants, or mixtures thereof. Also, liquid developers can be formulated from the product dispersion obtained with the process of the present invention. Moreover, the polymer particles of the present invention may be selected for formulating paints and chromatographic components.
Polymer or copolymer A can, for example, be a resin selected for superior charging or fusing properties in xerographic imaging and printing applications. Examples of the aforementioned polymer resins include homopolymers such as polyesters, poly(n-butyl methacrylate), poly(2-ethylhexyl methacrylate), poly(n-lauryl methacrylate), poly(stearyl methacrylate), poly(eicosane), and the like, and copolymers of the above with each other or with styrene or butadiene as, for example, poly(styrene-co-butyl methacrylate), poly(styrene-co-butadiene), and the like.
Polymer or copolymer segments B are selected for compatibility with the nonsolvent to achieve steric stabilization. For hydrocarbon nonsolvents, such as heptane, isooctane and Isopar mixtures available from Exxon, for example, B can include homopolymers such as polyethylene, poly(butadiene), hydrogenated poly(butadiene), poly(isoprene), and the like, or copolymers such as poly(ethylene-co-propylene). For polar nonsolvents such as methanol or ethanol, for example, B can include homopolymers such as hydroxypropyl cellulose, poly(N-vinylpyrrolidone), poly(vinylbutyral), poly(ethylene oxide), and the like, or other alcoholic soluble copolymers.
With further respect to the aforesaid A and B segments, the block stabilizer can be of the type AB, ABA, BAB, or the like, while the grafted stabilizer contains B side chains grafted onto A chains, or A side chains grafted onto B chains. These block or graft copolymers include commercially available materials such as Solprenes available from Phillips Inc., Kratons available from Shell Chemical Company, or Pluronics and Tetronics available from BASF, including dispersion polymerized particles, where the formation of 0.1 percent to 10 percent graft occurs during polymerization. For example, styrene polymerized in ethanol in the presence of hydroxypropyl cellulose, or poly(N-vinylpyrrolidone) generates about two percent of polystyrene grafted onto hydroxypropyl cellulose or poly(N-vinylpyrrolidone).
The precipitation of polymers is known for the isolation and purification of polymers, however, these processes usually provide undesirable large flocs or gummy materials usually unsuitable for toners or liquid inks. In contrast, with the processes of the present invention there are selected, for example, sterically stabilizing block or graft copolymers, enabling the formation of stable latex particles and avoiding the aforementioned prior art disadvantage.
More specifically, many process are known for the preparation of polymer particles for xerographic dry and liquid toners. Prior art processes for the preparation of toner size particles can generally be classified into two main areas: the mechanical melt blending, extrusion and jetting or micronization process, and the (2) chemical processes such as emulsion polymerization, suspension polymerization and dispersion polymerization which form particles directly from monomers.
The mechanical melt blending, extrusion and jetting process have several disadvantages which are overcome by the process of the present invention. Melt blending of particles in, for example, Banbury mixers, followed by extrusion in large commercial extruders is a semicontinuous process where the changeover from manufacture of one color of toner to another color involves the substantial waste of resin material needed to clean flush the blenders, extruders and jetting mill. In contrast, with the processes of the present invention the manufacture of a number of small batches of different toners, for example, for custom color or testing purposes without waste, or wherein waste of resin is minimized can be achieved. Also, there is a need for lower fusing temperature toners to extend fuser roll lifetime and reliability while decreasing energy costs. However, many lower melting resins cannot be jetted successfully, precluding their application for this need. The process of the present invention does not require jetting to prepare particles and therefore is believed to be better suited for lower melting resins. Further, the prior art multistep process of melt blending, extrusion and jetting consumes substantial time and is energy intensive. Also, with many of the prior art processes particles smaller than about 9 to 10 microns cannot be obtained without substantially greater processing time, classification and recycling steps. There is thus a need for economical processes, and processes wherein less energy is consumed that permit high resolution xerographic toners of from about 5 to 10 microns in average particle diameter, and preferably about 7 microns enabling, for example, improved copy quality of colored images. Melt blending, extrusion and jetting is not able in most instances to address this need economically.
To address the disadvantages of mechanical toner preparation via melt blending, extrusion and micronization or jetting, several chemical processes have been disclosed in the prior art, including emulsion polymerization, suspension polymerization, and dispersion polymerization. While these processes are useful for their intended purposes, they also have several disadvantages for the preparation of xerographic toners and inks.
Emulsion polymerization usually produces particles with average diameters of less than 1 micron. While this size is very satisfactory for liquid developers, it is too small for dry toners, where particles of 3 to 15 microns, and preferably 5 to 10 microns in average diameter are usually desired. The process of the present invention enables the preparation of polymer particles of an average diameter of from about 0.1 to 200 microns, thus it is more suitable for dry toner preparation than is emulsion polymerization.
Suspension polymerization is a very useful process for the preparation of toner resins to be used for the mechanical melt blending, extrusion and jetting process of toner particle manufacture. Unfortunately, the suspension polymerized particle size of 100 to 2,000 microns is too large to be suitable for direct use as either dry or liquid toners, thus there is a need for a process to convert the resin obtained from suspension polymerization into particles of 0.1 to 3 microns for liquid developers and 3 to 15 microns for dry toners. The process of the present invention satisfies these needs.
Dispersion polymerization is a useful process for the preparation of liquid or dry toner particles in the 0.1 to 15 average diameter micron range. However, the resin molecular weight and molecular weight distribution correlate strongly with particle size; the smaller particles have very much higher molecular weights. Therefore, it is difficult to prepare toner particles which possess both the desired particle size and the desired resin properties such as glass transition temperature, melting temperature, molecular weight,and molecular weight distribution. Furthermore, copolymerization dispersion processes are more difficult than homopolymerization, and have additional problems such as lower yield, multimodal size distributions, incomplete conversion, and the like. The process of the present invention avoids these problems associated with dispersion polymerization since the process of the present invention employs a preformed polymer in some embodiments. Thus it is possible to optimize the polymer properties separately from the particle size with the processes of the present invention in some embodiments For example, the present invention permits one to change the particle size of a dispersion polymerized particle to one more suitable for its application.
As a result of a patentability search there were located U.S. Pat. Nos. 3,257,341; 3,717,605; 3,876,610; 3,893,933 and 4,102,846. The '341 and '605 patents illustrate the process of polymerizing a monomer A in a solvent in the presence of a block or graft copolymer of monomers A and B. The polymer of A is not soluble in the reaction solvent, however, the polymer of B is soluble according to the teachings of this patent. Also the copolymer AB stabilizes the polymer A particles. Also, in column 1, lines 33 to 43, of the '341 patent the process is referred to as coprecipitation of polymer A and copolymer AB. However, the process described in these patents is known as dispersion polymerization, and involves the polymerization of monomers into polymer A. The process of the present invention does not involve any polymerization step. Thus, the dispersion polymerized particle products of the aforementioned patents may be selected as raw materials (polymer A and stabilizer AB) for the process of the present invention, enabling some of the advantages of the present invention as indicated herein. Since the products of the reactions cited in the preceding patents are dispersion polymerized particles, they contain grafted materials therein. However, an advantage of the processes of the present invention is that these dispersion polymerized particles of the prior art, which are in the 0.1 to 3 microns size range, can be dissolved and precipitated into larger particles with a diameter of form about 0.1 to 200 microns.
Furthermore, other references of background interest are U.S. Pat. Nos. 3,165,420; 3,236,776; 4,145,300; 4,271,249; 4,556,624; 4,557,991 and 4,604,338.
The process of the present invention provides advantages not usually available with mechanical melt blending, extrusion and micronization or jetting process, or from emulsion polymerization, suspension polymerization or dispersion polymerization particle formation reactions. In addition, the precipitation process of the present invention can be applied to preparation of both ink sized and toner sized particles, while many of the prior art processes are limited to one or the other.
The particles obtained with the processes of the present invention can be selected for toner compositions, including magnetic, single component, two component, liquid toners, and colored toner compositions. There are also provided in accordance with the present invention positively or negatively charged toner compositions comprised of the polymers obtained by the processes illustrated herein, pigment particles or dyes, and optional additive components such as metal salts of fatty acids, colloidal silicas, and charge enhancing additives. The toner, and developer compositions illustrated herein are useful in electrophotographic imaging systems, especially xerographic imaging methods. In addition, developer compositions comprised of the aforementioned toners and carrier particles can be formulated.
Moreover, toner and developer compositions containing charge enhancing additives, especially additives which impart a positive charge to the toner resin, are well known. Thus, for example, there is described in U.S. Pat. No. 3,893,935 the use of certain quaternary ammonium salts as charge control agents for electrostatic toner compositions. There is also described in U.S. Pat. No. 2,986,521 reversal developer compositions comprised of toner resin particles coated with finely divided colloidal silica. According to the disclosure of this patent, the development of images on negatively charged surfaces is accomplished by applying a developer composition having a positively charged triboelectric relationship with respect to the colloidal silica. Further, there are illustrated in U.S. Pat. No. 4,338,390, the disclosure of which is totally incorporated herein by reference, developer and toner compositions having incorporated therein as charge enhancing additives, organic sulfate and sulfonate compositions; and in U.S. Pat. No. 4,298,672, the disclosure of which is totally incorporated herein by reference, positively charged toner compositions containing resin particles and pigment particles, and as a charge enhancing additive alkyl pyridinium compounds, inclusive of cetyl pyridinium chloride.
Other prior art disclosing positively charged toner compositions with charge enhancing additives include U.S. Pat. Nos. 3,944,493; 4,007,293; 4,079,014 and 4,394,430.